This invention relates to the monitoring of vibration in rotary machines, and is particularly useful in monitoring vibration of bladed rotors such as found in compressors and turbines for gas turbine engines.
Vibration in rotary bladed machines, for instance compressors or turbines, is either tied or not tied to machine rotation. Non-tied vibration can be monitored by the use of a suitable detector probe mounted on the casing which normally surrounds the rotor blades. This is both simple and inexpensive to install on a machine and is effective in providing an indication of vibration in the rotating blades. Such detector probes are not effective, however, in the monitoring of vibration which is tied to machine rotation. This is because if a single blade on a rotating annular array of rotary blades is considered, that blade will always appear in the same vibrational position as it passes the probe detector. This is in contrast to non-tied vibration in which the blade appears in a position different to that which it occupied on its previous passage past the probe detector.
Tied vibration is conventionally monitored by the use of strain gauges or F.M. grids. In these methods, the strain gauges or F.M. grid magnets are actually located on the rotary parts of the machine. This being so, they are frequently difficult to install, and also tend to be expensive.